example: Explain setting network variables to constants.

example/example.org

@@ -325,6 +325,10 @@ tab
    | b | (and (not a) c) |
    | c | (not c)         |
 
+   Note that if you define the formula of a as 0, it will set a to 1,
+   because 0 is not #f.  For example, =(if 0 1 2)= evaluates to 1, and
+   not to 2.
+
    Here's the unsigned interaction graph of this network:
    #+NAME: simple-bn-ig
    #+BEGIN_SRC racket :results silent :var simple-bn=munch-sexp(simple-bn)